In a magnet-brush development process, developer powder, which includes magnetic material, stored in a developer vessel is conveyed to a development zone and attracted to a magnet roll. Image-bearing material, positioned adjacent the magnet roll, may be composed of a highly resistive polyester sheet, photoconductive selenium, an electrically insulating film overlying a layer of photoconductive cadmium sulfide disposed in an insulating binder, a thin film of polyvinylcarbazole or poly-N-vinylcarbazole, a layer of the mixture of photoconductive zinc oxide and an insulating resin binder, or the like, as known in the art.
The developer powder is supplied from the developer vessel through a gap of predetermined size onto the magnet roll and, according to the rotation of the magnet roll, the developer powder rotates or tumbles along the roll to the development zone. At least at the development zone, the developer forms a magnet-brush on the magnet roll and the magnet-brush rubs the surface of the image-bearing material to adhere the toner material of the developer powder to electric pattern images on the surface. For purposes of this application, electric pattern images include electrostatic images, capacitive images, and electrically conductive images. For the convenience of explanation, the latent electrostatic images will be used as representative in this specification.
In the development, there has been used an admixture of ferromagnetic carrier particles and toner particles. The ferromagnetic carrier particles are resin-coated-iron beads and the toner particles are a mixture of pigment and binder. The carrier particles and the toner particles are triboelectrically charged to the opposite polarity by blending them. The materials of the carrier particles and the toner particles are selected to cause a charge on the toner opposite to the charge of the electrostatic latent image on the image-bearing material. The admixture is stored in the developer vessel in which the toner particles adhere to the surfaces of the carrier particles by the triboelectric charge and is then conveyed on the surface of the magnet roll as the roll rotates. The admixture forms a magnet-brush at the development zone and, when the brush rubs the latent image, the toner particles adhere to the latent image by the electrostatic attraction force between the charge of the latent image and the charge of the toner, but the carrier particles remain on the magnetic roll by the magnetic attraction force between the carrier and the roll. After the development the admixture, less the adhered toner, returns to the developer vessel and is supplied new toner.
On the other hand, a single component magnetic toner has been improved to be used in the magnet-brush development and has the advantage that it is not necessary to use the carrier particles or to mix them. Although such a magnetic toner is referred to as "single component" or "one component," the name does not mean that the toner consists of only one component, but the toner comprises mainly one kind of particles composed of fine magnetic particles, organic binder, pigment, carbon black and flow agents. No so-called "carrier" is required.
The structure of the magnet roll is well-known and is shown, for example, in Anderson, U.S. Pat. No. 3,455,276, Samuels et al, U.S. Pat. No. 4,003,334 and Yamashita et al, U.S. Pat. No. 3,828,730. Anderson refers to a magnet roll as a magnetically responsive powder applicator, which comprises a shaft of high magnetic permeability material, a plurality of elongate, generally sector-shaped in cross section, magnetic members formed of fine grain, permanent magnet material dispersed in a non-magnetic matrix, which members are positioned to define a circular array around the shaft, the alternate, outer faces of adjacent members being oppositely polarized, and a uniform non-magnetic hollow cylindrical sleeve positioned over the magnetic members and extending axially relative to the shaft. In Anderson the sleeve and the magnet members rotate relatively.
Samuels et al and Yamashita et al disclose the use of a one-body ceramic magnet, instead of elongate, generally sector-shaped in cross section, magnetic members formed of rubber magnet material. The Samuels et al magnet roll has an insulating sleeve or shell and Yamashita et al has two adjacent magnetic poles of the same polarity. In the magnet-brush development, it is necessary to make substantially constant the amount of the developer powder supplied to the development zone on the magnet roll for maximum clarity. In order to control the amount of the developer powder, a doctor blade is used close to an exit of the developer vessel and is positioned adjacent and in spaced relation to the magnet roll, forming a narrow spacing between the doctor blade and the shell surface of the magnet roll, hereafter called "doctor spacing." When the doctor spacing is narrower than the spacing between the shell surface and the image-bearing surface, the developer powder is not sufficient to develop the latent image on the image-bearing material. On the other hand, when the doctor spacing is broader than the spacing of the development zone, the developer powder tends to accumulate at the development zone and the accumulation grows with time. It is very important to the sensitivity of the development process to adjust the doctor spacing to control the amount of developer powder at the development zone.
By contrast, Nishihama et al, U.S. Pat. No. 4,081,571 proposes an electrostatic development method in which the minimum thickness of developer layer on a magnet roll is larger than the spacing between the magnet roll and the image-bearing material to form a developer accumulation, or reservoir zone, of the developer powder upstream of and adjacent the development zone. According to Nishihama et al, a roll-like permanent magnet of the magnet roll rotates in the opposite direction to the rotating direction of a developer-carrying member, such as a cylindrical, non-magnetic shell, of the magnet roll so that the developer powder in the reservoir is magnetically disturbed. Furthermore, since the reservoir of the developer powder is growing larger with time, at the end of every developing cycle, the developer powder in the reservoir should be carried away by the developer-carrying member, or a drum-shaped image-bearing member should have a groove of a relatively small diameter at a position of the surface of the image-bearing member to allow the reserved developer powder to pass through on a cyclic basis. The change of the amount of the developer powder in the development zone tends to cause developed images to be uneven.